1. Field of the Invention
The present invention relates to an electron beam exposure system for deflecting and focusing an electron beam and for forming a fine pattern on a sample (semiconductor wafer or the like), and more particularly relates to an electron beam exposure system having a low aberration focusing deflector for realizing a high resolution image.
2. Description of the Related Art
In recent years, simultaneously with a high integration of semiconductor integrated circuit devices, an election beam exposure system has been requested for an increase in throughput and the realization of a high resolution image.
The electron beam exposure system scans an electron beam on a sample and obtains a desired pattern by deflecting the electron beam using a plurality of electromagnetic deflection coils (electromagnetic deflector).
The plurality of coils are separated into two portions (X, Y) in accordance with the scanning directions, and in each portion, the deflection coils are connected in series. The electron beam forms a desired shape, after which it is condensed by a condensing lens and projected onto a sample by a projection lens.
The strength of a lens is determined so that an image of a first rectangular aperture formed at first is obtained on a second rectangular aperture for forming the shape of an electron beam and on a sample; and so that a crossover image is combined on a round aperture. There are no specific conditions regarding the strength of the condensing lens and the projection lens, and a combination thereof to image the electron beam on a sample is necessary.
The resolution of a fine pattern is directly affected by a blurring of the electron beam itself. This is a characteristic of the exposure system and is called aberration. There are two types of aberration, an on axis aberration which occurs on an optical axis regardless of the beam deflection, and a deflecting aberration which is caused by deflection.
The on axis aberration is caused by a characteristic of the electromagnetic lens and occurs regardless of the deflector. Namely, the amount of on axis aberration is determined only by the focusing route to the point where the beam arrives at the sample, and the beam aperture angle when the electron beam enters the sample, i.e., the angle of incidence is proportional to the value. Generally, using half of the angle of incidence (.alpha.), the aberration is expressed as follows. EQU Spherical aberration C.sub.s =k.sub.1 .alpha..sup.3 EQU Chromatic aberration C.sub.c =K.sub.2 .alpha. .DELTA.E
where .DELTA.E designates a fluctuation of the electron energy emitted from an electron gun. The smaller .DELTA.E is, the smaller the chromatic aberration is, and most of the aberration on the optical axis is spherical aberration.
When .alpha. decreases, the spherical aberration decreases in proportion to .alpha..sup.3. Namely, resolution of the electron beam increases in accordance with the reduction of .alpha.. The value .alpha. of half of the aperture angle is determined in accordance with a design of the round aperture, and also the lower limit of the electric current density determines the lower limit of the area of the round aperture, therefore the value .alpha. cannot be reduced beyond a certain value.
The deflection aberration is generated by beam deflection using the deflector, and includes components in proportion to a value d of the deflection, d.sup.2 and d.sup.3. Some of these aberrations cannot be corrected. Hitherto, aberrations which could not be corrected were minimized, and the aberrations which could be corrected were corrected by the design of the deflectors to reduce the aberrations as much as possible.
On the other hand, half the angle of the aperture can be changed without changing the lower limit of the electric current applied to the sample, by changing the strength ratio of the projecting lens and the condensing lens. If the strength of the projecting lens is reduced and the strength of the condensing lens is increased, then half the angle of aperture of the condensing lens having a comparatively large condensing ratio is reduced, and half the angle of aperture of the projecting lens having condensing ratio of approximately 1 is reduced.
By adjusting the lens strength ratio, the aberration on the optical axis can be reduced. However, since the strength of the projecting lens is large, the beam deflected by the deflector is moved in the direction of the optical axis, and the deflecting efficiency is reduced.
Further, when the half angle .alpha. of aperture for the lens is constant, the spherical aberration can be reduced. Since spherical aberration occurs when the beam passes through the periphery of the lens, improvement of the spherical aberration is accomplished simply by short focusing so that the shape of the lens is reduced.
Since the beam passing through the periphery of the lens passes through a shorter route due to the short focusing, the spherical aberration reduces in accordance with the reduction ratio of the shortened route. Further, blurring of the beam caused by the space charge effect can be reduced by short focusing. In this case, the deflection domain is also reduced.
Although the above two methods cannot avoid the reduction of the deflection domain, the aberration on optical axis can be reduced. In the case of a variable rectangular beam wherein the beam current changes greatly, when the rectangle is large, namely, when the beam current is high, the blurring of the beam caused by the space charge is greater than the spherical aberration.
For this reason, short focusing is especially effective. If the reduction of the deflection domain is allowed, the best focusing deflector is obtained by using short focusing instead of changing of the lens strength ratio.
A deflector located in a short focusing lens must be small so that the deflector is included in an inner radius of the polepiece (magnetic pole) of the lens.
Further, in order to increase the deflection efficiency as much as possible, the magnetic field generated by a coil must be strong. For this reason, miniaturization of the coil must be made in consideration of the drifting of the beam position caused by the heating and inductance of the coil.
Before the advent of short focusing, the radius of curvature of the coil was, for example, 20, 24 millimeters or the like, and the focal distance was approximately 200 millimeters.
However, it was known that when the coil is miniaturized, aberration becomes exceedingly large. This phenomenon could not be solved by the prior art theory.